A pR351 plasmid (Tc Ap Cb) conferring drug resistance of naturally occurring E. coli strain was examined. Conjugation and transduction experiments have indicated that this plasmid is R plasmid aggregate consisting of three independent plasmids: a) conjugative plasmid pR351 A (SuTc) fi- (F) belonging to incompatibility group L, b) conjugative plasmid pR351 B (SuApCb) fi- (F) belonging to incompatibility O, and c) non-conjugative plasmid pR351 C (ApCb). The existance of these plasmids in pR351 plasmid aggregate was confirmed by the agarose gel analysis of plasmid DNA isolated from the lysates of E. coli J53 transconjugants carrying pR351 A or pR351 B plasmids and from E. coli K12 C600 transductant carrying pR351 C plasmid. Molecular mass of these plasmids was found to be 55.60 and 3 Mdal respectively. The non-conjugative pR351 C plasmid could be mobilized by Col B and F factors. Our findings have indicated that two fi- (F) R plasmids can stably coexist in R plasmid aggregate.